This invention relates generally to improvements in hip joint prostheses, particularly with respect to an improved acetabular cup bearing component constructed from a relatively hard and relatively high strength ceramic material. The ceramic acetabular cup is designed for direct articulation against an associated femoral head component formed from a compatible ceramic or metal material, with minimal component wear to achieve substantially optimal prosthesis service life. The present invention thus permits elimination of a conventional polymer-based bearing insert thereby also eliminating undesirable wear debris associated therewith.
Hip prostheses generally comprise a femoral component adapted for fixation to an upper end of the patient's femur, and defining a generally ball-shaped head for articulation relative to an acetabular cup component which is adapted in turn for seated fixation within the patient's acetabulum. A polymer-based bearing insert such as a cup-shaped component formed from a typically high density or high molecular weight polyethylene (PE) or the like is normally fitted between the femoral head and the acetabular cup to accommodate smooth and relatively low-wear articulation between these components.
However, clinical studies have shown that significant wear debris can be generated and released by the polymeric bearing insert over a period of time, and further that a principle contributing factor to implant failure is osteolysis attributable at least in part to the presence of such polymer-based wear debris. More particularly, such studies have shown that PE wear debris released into peri-implant tissues appears to elicit a deleterious biological reaction, incorporating foreign body giant cell and macrophage cell responses leading to undesirable bone resorption, with eventual loosening and failure of the prosthetic implant. As a result, alternative prosthesis constructions have proposed improvements in and to the polymer-based bearing insert, such as the use of heavily cross-linked polyethylene materials. Other alternative prostheses have been proposed using rigid-on-rigid components, such as ceramic-on-ceramic or metal-on-metal, thereby eliminating the polymer-based bearing insert and wear debris associated therewith.
In general, ceramic hip prosthesis components have shown promise for use in a ceramic-on-ceramic or alternately in a ceramic-on-metal articulating interface, thereby completely eliminating the polymer-based bearing insert. Such prosthesis constructions, when formed with a good surface finish and conformal surface geometry, have demonstrated a relatively low coefficient of friction and resultant substantial reduction in component wear in comparison with ceramic-polymer or metal-polymer articulatory interfaces. However, the major limitation on the use of ceramic components particularly such as alumina-based ceramic materials has been an unacceptably high rate of brittle fracture occurring within a post-surgical follow-up period ranging from a few months to several years. In this regard, ceramic materials generally exhibit relatively low toughness and are thus prone to brittle fracture.
U.S. Publication US 2003/0153984 discloses an improved ceramic material for use in joint prostheses, such as hip prostheses, wherein a ceramic-on-ceramic or a ceramic-on-metal articulatory interface is defined. The improved ceramic material comprises a doped silicon nitride (S3N4) having relatively high hardness, tensile strength, elastic modulus, lubricity, and fracture toughness. Specifically, the improved doped silicon nitride ceramic has a flexural strength greater than about 700 Mega-Pascal (MPa) and a fracture toughness greater than about 7 Mega-Pascal root meter (MPam0.5). This high strength and high toughness doped silicon nitride ceramic achieves ultra-low wear over an extended service life, with dramatically reduced risk of brittle fracture.
In addition, U.S. Pat. No. 6,846,327 discloses improved ceramic materials for bone graft applications, wherein the ceramic material is designed to mimic structural characteristics of natural patient bone by including first and second regions of comparatively lower and higher porosity to respectively mimic natural cortical and cancellous bone structures. The preferred ceramic materials disclosed exhibit a flexural strength greater than about 500 Mega-Pascal (MPa) and a fracture toughness greater than about 5 Mega-Pascal root meter (MPam0.5). In use, the relatively low porosity region of the ceramic material provides high structural strength and integrity, whereas the higher porosity region is suitable for bone ingrowth to achieve secure and stable implant affixation.
The present invention comprises an improved hip joint prosthesis particularly wherein the acetabular cup component thereof is constructed from an improved high strength and high toughness ceramic material as disclosed, e.g., in U.S. Publi. US 2003/0153984 and/or U.S. Pat. No. 6,846,327.